1. Field of the Invention
The invention pertains to systems for transmitting and/or receiving electromagnetic signal radiation, which systems include electromagnetic cavity resonators.
2. Art Background
Electromagnetic cavity resonators, also called resonant cavities, are devices which include cavities (chambers) enclosed by electrically conductive walls. The geometries and dimensions of these cavities are chosen so that particular electromagnetic waves, having specific frequencies/wavelengths, resonate within the cavities, i.e., undergo reflections from the walls of the cavities to produce standing wave oscillations.
A resonant cavity having a configuration which (as discussed below) is of particular relevance to the present disclosure is the resonant cavity depicted in FIG. 1. As shown, this resonant cavity includes an outer cylindrical wall and an inner, coaxial, solid cylinder, both of which are, for example, of copper, and both of which are, for example, circular in cross-section (as depicted). For an electromagnetic wave propagating parallel to the longitudinal axis of the resonant cavity, having a radial electric field, E, and a circular magnetic field, B, resonance is achieved at a wavelength (within the resonant cavity), .lambda., which is equal to twice the length, L, of the resonant cavity.
A figure of merit useful in characterizing the frequency selectivity of a resonant cavity, i.e., the ability of the cavity to sustain electromagnetic oscillations at frequencies which are slightly off-resonance, is the quality factor, Q, of the cavity. That is, if, hypothetically, one were to insert a vanishingly small electrical wire, producing a minute amount of power dissipation and having a loop on its end, through an opening in an appropriately chosen surface of the cavity, and flow alternating current, at frequencies close to the resonant frequency, through the wire, electromagnetic waves having corresponding frequencies would be produced within the cavity. In this hypothetical scenario, the strengths of the waves within the cavity are inferable by inserting a second vanishingly small wire, also producing a minute amount of power dissipation and also having a loop on its end, into the cavity and measuring the electrical powers associated with the alternating currents induced in the second wire. If one were to plot these electrical powers (associated with the induced currents) versus frequency, f, then a plot like that shown in FIG. 2 would be obtained. As expected, the maximum power occurs at the resonant frequency, fo, with power rapidly decreasing at frequencies off resonance. In this regard, the quality factor, Q, of the resonant cavity (per se) is equal to fo/.DELTA.f, where .DELTA.f (see FIG. 2) denotes what is conventionally termed full width at half power, i.e., the width of the frequency range over which the electrical powers associated with the induced currents have fallen to one-half the peak power.
Significantly, as is known, the Q of a resonant cavity (per se), and thus the frequency selectivity of the cavity, is equal to 2.pi.fo.multidot.W/P, where W denotes the electromagnetic energy stored in the cavity and P denotes the average electrical power dissipated in the walls of the cavity. That is, if the walls of the resonant cavity were perfect electric conductors, i.e., the walls were impenetrable to electric fields and exhibited no electrical resistance, then only the corresponding resonant oscillation could be maintained within the cavity, and therefore Q would be infinite. However, if the walls are imperfect conductors (as is always the case with conventional electric conductors), then the electric field associated with a slightly off-resonant oscillation will penetrate the walls (at least slightly) and, as a consequence, it now becomes possible for the off-resonant oscillation to be maintained. Such penetration will induce currents in the walls which will serve to expel the field and preclude electromagnetic energy accumulation within the cavity at the off-resonant frequency. However, because the imperfectly conducting walls exhibit electrical resistance, electrical power will be dissipated in the walls, and therefore the currents will be less than are needed to expel the field. Consequently, the off-resonant oscillation will be maintained, to the degree that power is dissipated in the walls (and provided the dissipated power is replenished). Thus, it is power dissipation which accounts for the presence of off-resonant oscillations and finite Qs.
As is known, the intensity of an alternating electric field within a normal (conventional) electric conductor decays exponentially with depth, and the particular depth at which the field decays to 1/e of its maximum value where e is the base of natural logarithms having the approximate value 2.71828, is called the skin depth. As is also known, essentially all the power dissipation, described above, occurs within the skin depth, and it is the corresponding electrical resistance, called the surface resistance (the real component of the surface impedance), which is responsible for this power dissipation. In this regard, it can be shown that the Q of a resonant cavity is inversely proportional to the surface resistance of the cavity. In particular, in the case of the coaxial resonant cavity depicted in FIG. 1, it can be shown that the Q of the cavity is approximately equal to ##EQU1## where a and b are the radii, and R.sub.a and R.sub.b are the corresponding surface resistances, of, respectively, the inner solid cylinder and the outer cylindrical wall, and Z.sub.o is the real component of a characteristic impedance of the resonant cavity. If, for example, R.sub.a /a.sup.2 is substantially larger than R.sub.b /b.sup.2, then the Q of the cavity is approximately equal to ##EQU2##
Significantly, resonant cavities exhibiting relatively high Qs are employed as narrow bandpass filters in systems for transmitting and/or receiving radio-frequency and microwave-frequency electromagnetic signal radiation, such as cellular radio systems. In this regard, as is known, the frequency spacing between adjacent signal channels in cellular radio systems is limited by the Qs of currently available resonant cavities. That is, smaller frequency spacings, in both present and planned systems, are desirable, indeed, in some cases, essential. However, these smaller frequency spacings can only be achieved by employing resonant cavities which exhibit correspondingly higher Qs. While the Q of a cavity can be increased by increasing the dimensions of the cavity, the Qs needed to achieve significantly smaller frequency spacings are so high that the corresponding cavities would have to be impractically large.
An attempt has been made to achieve higher Qs, without increasing cavity dimensions, by employing a material which was assumed to exhibit a substantially lower surface resistance than conventional materials, such as copper. (See, e.g., Eq.(2), which indicates that a reduction in R.sub.a results in a corresponding increase in Q.) That is, a coaxial resonant cavity, of the type depicted in FIG. 1, has been fabricated, in which the central copper cylinder was replaced by a cylinder which included yttrium barium copper oxide (YBa.sub.2 Cu.sub.3 O.sub.7), one of a newly discovered class of superconducting cuprates, i.e., cuprates which exhibit zero electrical resistance to DC electrical current. In this regard, the YBa.sub.2 Cu.sub.3 O.sub.7 cylinder was fabricated, conventionally, by initially forming a mixture of precursors of the superconducting material, i.e., copper oxide, barium carbonate and yttrium oxide. This mixture was ground, using a ball mill, into a powder in which the powder particles were typically 40 micrometers (.mu.m) in size. The powder was then mixed with a few drops of deionized water to form a paste, which was placed in a mold and subjected to a pressure of 40,000 pounds per square inch (psi). After being removed from the mold, the resulting body was sintered (heated) in an oxygen atmosphere at 900 degrees Centrigrade (C.) for four hours, which served to convert the precursor materials to YBa.sub.2 Cu.sub.3 O.sub.7, and then annealed in an oxygen atmosphere at a temperature which was reduced from 500 degrees C. to room temperature at a rate of 1 degree C. per minute. (Regarding this conventional processing see G. E. Peterson et al, "Coaxial Lines and Cavities Containing High T.sub.c Superconducting Center Conductors," Proc. IEEE Princeton Section Sarnoff Symposium, Sept. 30, 1988.)
As is known, the newly discovered superconducting cuprates exhibit relatively high critical temperatures, T.sub.c (the temperature above which the material ceases to be superconducting), i.e., exhibit T.sub.c S higher than 77 Kelvins (the boiling point of liquid nitrogen). Significantly, the cylinder of YBa.sub.2 Cu.sub.3 O.sub.7, fabricated using the conventional processing, described above, exhibited a T.sub.c of 90 Kelvins.
Upon immersing the resonant cavity, containing the cylinder of YBa.sub.2 Cu.sub.3 O.sub.7, in liquid nitrogen, it was hoped that the cavity would exhibit a substantially higher Q (by virtue of a lower surface resistance) than a similar cavity immersed in liquid nitrogen, in which the central cylinder is of copper. While the superconductor-containing cavity did exhibit higher Qs than a corresponding copper-containing cavity, at 77 Kelvins and at frequencies ranging from about 5 to about 50 megahertz (MHz), these Qs were, unfortunately, typically no more than about 50 percent higher (and the corresponding surface resistances were no more than about 33 percent lower), which is less than desired. (Regarding the Qs of the superconductor-containing cavity see G. E. Peterson et al, supra.)
It should be noted that the conventionally fabricated cylinder of YBa.sub.2 Cu.sub.3 O.sub.7, referred to above, not only resulted in disappointingly low Qs at 77 Kelvins, but also proved to be fragile (i.e., exhibited flexural strengths less than about 50 megapascals (MPa)), making handling difficult. Moreover, the conventional methods used to fabricate the cylinder proved incapable of producing YBa.sub.2 Cu.sub.3 O.sub.7 bodies having relatively complicated shapes, e.g., helical shapes, which, as discussed below, is a significant drawback.
Thus, those engaged in developing electromagnetic-radiation transmission and receiving systems have sought, thus far without success, relatively small-sized resonant cavities which exhibit relatively high Qs at a temperature of, for example, 77 Kelvins.